1. Field of Invention
This invention relates to an alignment system for a lithographic apparatus, and a lithographic apparatus having such an alignment system, and more particularly to an alignment system that can detect the position of an alignment mark using at least two separate wavelength signals.
2. Discussion of Related Art
Lithographic apparatuses are essential components for the manufacture of integrated circuits and/or other micro-devices. With the aid of such an apparatus, different mask patterns are successively imaged at a precisely aligned position onto a substrate such as a semiconductor wafer or an LCD panel. The substrate may undergo physical and chemical changes between the successive images that have been aligned with each other. The substrate is removed from the apparatus after it has been exposed with the image of a at least one mask pattern, and, after it has undergone the desired process steps, the substrate is placed back in order to expose it with an image of a further mask pattern, and so forth, while it must be ensured that the images of the further mask pattern and the subsequent mask patterns are positioned accurately with respect to theat least one already exposed image on the substrate. To this end, the lithographic apparatus is provided with an alignment system with which alignment marks on the substrate are aligned with respect to alignment marks on the mask.
A lithographic apparatus may not only be used for the manufacture of ICs but also for the manufacture of other structures having detailed dimensions of the order of 1 micrometer, or smaller. Examples are structures of integrated, or plenary, optical systems or guiding and detection patterns of magnetic domain memories, micro-electromechanical systems (MEMS), and structures of liquid crystal display panels. Also in the manufacture of these structures, images of mask patterns must be aligned very accurately with respect to images already exposed onto the substrate.
The lithographic apparatus may be a stepping apparatus or a step-and-scan apparatus. In a stepping apparatus, the mask pattern is imaged in one shot on an exposure area of the substrate. Subsequently, the substrate is moved with respect to the mask in such a way that a subsequent exposure area will be situated under the mask pattern and the projection lens system and the mask pattern is imaged on the subsequent exposure area. This process is repeated until all exposure areas of the substrate are provided with a mask pattern image. In a step-and-scan apparatus, the above-mentioned stepping procedure is also followed, but the mask pattern is not imaged in one shot, but via scanning movement. During imaging of the mask pattern, the substrate is moved synchronously with the mask with respect to the projection system and the projection beam, taking the magnification of the projection system into account. A series of juxtaposed partial images of consecutively exposed parts of the mask pattern is imaged in an exposure area. After the mask pattern has been completely imaged in an exposure area, a step is made to a subsequent exposure area. A possible scanning procedure is described in the article: “Sub-micron 1:1 Optical Lithography” by D. A. Markle in the magazine “Semiconductors International” of May 1986, pp. 137-142.
U.S. Pat. No. 5,243,195 discloses an optical lithographic apparatus provided with an alignment system and intended for the manufacture of ICs. This alignment system comprises an off-axis alignment unit for aligning a substrate alignment mark with respect to this alignment unit. In addition, this alignment system comprises a second alignment unit for aligning a substrate mark with respect to a mask mark via the projection lens (TTL). Alignment via the projection lens (on-axis alignment) is frequently used in many current generation of optical lithographic appalithographic apparatuses and provides the advantage that the substrate and the mask can be aligned directly with respect to each other. When the off-axis alignment method is used, the baseline offset as described in U.S. Pat. No. 5,243,195 must be taken into account. However, with the continued decrease in the size of components on ICs and the increase in complexity, on-axis alignment systems have proven to be difficult to improve sufficiently to achieve the required precision and accuracy.
In connection with the increasing number of electronic components per unit of surface area of the substrate and the resultant smaller dimensions of these components, increasingly stricter requirements are imposed on the accuracy with which integrated circuits are made. The positions where the successive masks are imaged on the substrate must therefore be fixed more and more accurately. In the manufacture of new-generation ICs with smaller line widths, the alignment accuracy will have to be improved or, in other words, it must be possible to detect smaller deviations so that the resolving power of the alignment system must be increased. On the other hand, stricter requirements must also be imposed on the flatness of the substrate due to the required higher numerical aperture (NA) of the projection lens system in the case of decreasing line widths. The depth of focus of this system decreases as the NA increases. Since some image field curvature occurs at the desired relatively large image field of the projection lens system, there is hardly any room left for unevenness of the substrate. To obtain the desired flatness of the substrate, it has been proposed to polish the substrate by a chemical mechanical polishing (CMP) process between two consecutive exposures with different mask patterns in the lihographic apparatus. However, this polishing process affects the accuracy of the on-axis alignment method. In this method, a grating is used as a substrate alignment mark and the sub-beams diffracted in the first order by this grating are used for imaging the substrate mark on the mask mark. In this process, it is assumed that the substrate is aligned correctly with respect to the mask when the point of gravity of the substrate grating mark is aligned with respect to the point of gravity of the mask alignment mark. In that case it has been assumed that the point of gravity for each grating mark coincides with the geometrical center of the grating. However, the CMP process renders the substrate grating mark asymmetrical so that this alignment method is no longer reliable. In addition, the various processing steps contribute to changes in the alignment marks including introducing asymmetries and changes in the effective depth of the grooves of the substrate grating marks. Since the signal strength of monochromatic light reflected from such a phase grating varies periodically with the depth of the grooves, the processing can render grating marks undetectable in some cases or provide only a weak signal in other cases. This leads to a decrease in the robustness of the alignment system in that there are cases when an expected alignment detection cannot be made due to a loss of signal strength. This can also lead to a decrease in alignment precision if a weak signal is used to determine the position of the alignment mark. One approach to alleviate this problem is to use two separate wavelengths to illuminate and detect the position of the alignment mark on the substrate. However, the use of light sources in such systems that are in the visible region of the spectrum, e.g., a red and a green laser, results in situations where the signals at both wavelengths are weak, thus leading to problems with robustness and precision of detection of the alignment marks on the substrate.